Does anyone know why, or better yet where I can read an explanation for why rotary exhaust is so much hotter than piston engine exhaust?

Matt

 

The rotary has to open the exhaust port and dump the exhaust out sooner than a piston motor. Since the exhaust is coming out sooner, the combustion gasses have not been expanded fully. All engines expand a hot gas and convert the heat energy into work. Since the rotary opens up the exhaust and dumps it sooner, then there is still a lot of heat energy in the exhaust, and thus a lot of temperature. This is also the reason a rotary engine will not acheive the gas mileage (thermal efficiency) of a piston engine, it is because there is a lot of wasted energy going out the exhaust of the rotary.

The complex motion of the rotor, eccentric shaft, and gears mandate that the exhaust be dumped sooner on the rotary. If the exhaust gasses are held in longer, then they start to do negative work on the face of the rotor (they push the wrong way on the e-shaft,) meaning you loose power if you try and hold the exhaust in longer by delaying the exhaust port opening timing.

 

i think it might have to do with the fact that it is in a constant firing cycle, unlike the piston engine with 4 strokes.

 

Speedturn got it. It's hot relative to a 4-stroke piston engine.

With a piston engine the exhaust gases have to flow around the valve head, then flow around the valve stem, then make a right angle turn past some water jacket to get out.

That's also why a rotary is way louder than a boinger.

 

Thanks speedturn, you answered it perfectly. I was hoping to get a little of the thermodynamics of the situation along with the answer.

Sureshot, not only do the gasses have a more complicated exit in a piston engine, but since they exit at a lower pressure less sound energy is generated.

Not sure if you guys saw this or not, but BMW just came out with a turbosteamer prototype that uses a steam power circuit heated by exhaust gas to reclaim wasted energy. http://www.speedtv.com/articles/auto...hnology/21159/ It makes thier 4 cylinder engine like 15 percent more efficient. It seems like a similar system would be excellent coupled with a rotary engine.

Matt

 

Not sure where you guys get your information from but just to let you know that the power stroke of a rotary is 270 degrees vs. 180 degrees in a piston so they're actually keeping the stroke longer than the piston.....

 

That's geometrically impossible..

I might believe 160 vs. 120.

 

what's geometrically impossible? rotary or piston?

 

Plutos answer is actually a trick answer. The E shaft spins 270 degrees per phase (intake, compression, power, exhaust), but the port timing is still worked out in 180 degree increments of rotor phasing. The E shaft spins faster, in relation to, the rotor and its 4 "strokes". This is how you can figure that a rotary opens its exhaust "valve" earlier than a typical piston engine.

 

Pluto, the length of the power stroke is mostly unrelated to gas expansion when you are comparing engines. An internal combustion engine produces power by expanding a heated gas, the further the gas is expanded, the lower it's pressure and temperature become and the more energy is extracted from it.

Matt

 

This also has something to do with it. Each rotor exhaust port has a continuious exhaust energy while a 4 stroke piston will have the exhaust pulses come every other rotation of it's crank shaft per cylinder. This leaves a slight cooling off period for the piston engine in each cylinder where as a rotary will have none.

 

I think the fact that oil is injected raises temps a bit

 

The noise level as well as the exhaust temperature is just as stated . There is no interrupted flow (obstruction--- Valve and stem) in the exhaust port. It is a straight shot. The valves remove alot of heat from a 4 cycle engine through the seat and guides. The high noise level is for the same reason. Think of a turbo in the exhaust of a 962-935 Porsche race car( Or any tubo) . They were producing up to 800Hp but ran rather quietly without mufflers because the turbo act as a sort of muffler. A second gen turbo rx7 producing 280-300hp with no mufflers is alot quieter than a 300 hp Pport with the same exhaust.

 

Thats correct sccagt3. It's important to note though that a turbo makes a car quieter not just because it is an abstruction, but it removes energy from the exhaust (which it uses to compress intake air) by further expanding and cooling exhaust gas. Sound and heat are both part of the kinetic energy of the exhaust, the other part is the velocity it is moving at and it's pressure.

Ordinary obstructions are removing energy too, because the exhaust is slowed by them, reducing thier velocity (and overall kinetic energy) and increasing the opportunity for heat transfer (another reduction of the energy of the exhaust).

I'm not trying to be a smartass, just to have an engine discussion based on thermodynamics.

Matt

 

Didn't you just say that the further the gas is expanded, the lower the pressure and temperature? Isn't longer power stroke allows the energy to transfer into mechanical energy further which in terms create less losses (heat)?

 

Yes I did, and no. In order to expand the gas farther you have to increase it's volume. Comparing the lengths of the power strokes (in crank rotation) between the 2 engines is not a direct comparison. The rotary power stroke may involve more crank rotation, but it takes more crank rotation for it to change it's volume. The mechanisms are so different you have to compare exactly what the engine is doing to the gas.

Matt

 

I was only trying to give a simple explanation. I thought an in depth Thermo discussion was not needed. Where did you do your SAE? My son is doing that at Duke when he can afford the time .

 

damn interesting thread....

 

Yup.

And the funny thing about a turbo making exhaust quieter... My roommate started an argument with me: that a turbo will make it louder since it is going to be forcing more air through the engine, hence more exhaust, and more sound.

I even brought up the point of a WRX STi (2.5l turbo) with full exhaust (no cats, nothing) compared to my current Tiburon (2.0l NA) with a similar exhaust system, and a single high flow cat, and my car was very noticibly louder. He didn't have a good response to that, but ended the debate with "turbos just make it louder, more exhaust is louder" and left the room.

I think I'll point him to this thread.

 

Glad to help.

 

Tell me about it. I just modified my NA S5 exhaust sleeves in my s5 rebuild. I still have the stock exhaust but now it's much louder with defusers removed. At first I thought I had and exhaust leak. Nope. I never expected the db's to raise that much when doing that modifcation.

 

I was at Kettering University for 3 cars, 2002, 03, 04. I highly recommend it. However, nobody has the amount of free time it requires, you just have to do it and make time. I learned far more doing that than I did in any of my classes.

 

Matt,
That was my point of argument from the beginning, it wasn't the longer duration of the power stroke that makes the exhaust run hotter. I was correcting speedturn that the exhaust doesn't come out sooner than the piston engine (which is why I stated that it was 180 degrees on piston vs. 270 degrees on a rotary). The reason for higher exhaust temp from my understanding is that the inherent design flaw of the rotary engine where it doesn't burn fuel efficiently. If you look at the length and the surface area of the rotor in respect to how it creates the flame front vs. piston engine for the power stroke, you'll see that it takes much much much longer time to burn the fuel completely (which is why by having a 270 degrees power stroke actually help offset that problem, and also 3 "timed" sparks per power stroke to help burn the fuel more efficiently). Eventhough, most of the a/f mixture were converted into mechanical energy during the power stroke, there're still excessive amount of unburn fuel that exit to the exhaust which in terms will convert into heat creating a higher EGT.

 

This is an awesome discussion, thought I'd add some of my ideas/theories too.

In reference to how the rotary doesn't as efficiently use the energy of the exhaust gasses, there are ways to reclaim it (similar to the BMW's innovation). First we all know, the turbo which takes the still expanding gasses and transfers that energy to compression, which in turn provides even more compressed gases coming out of the exhaust.

Second, any kind of secondary turbo-like driven electric motor that allows the accesories/alternator to run off of it while not creating too much back pressure and hindering the turbo's operation.

The third would be a similar idea to the second except the accessories would be driven off of a device which harvests pure heat energy from the exiting gasses instead of direct mechanical motion, to convert it into mechanical motion for the accesories.

I beleive mazda has explored this with one of its newer concepts (SENKU?), in mating an electric hybrid type starting motor/generator to the rotary.

 

"device which harvests pure heat energy"

build one of those, and you will be a billionaire the next day. there is no such device. the only current feasible way to harness heat energy is by expansion/contraction of a fluid. this is why many systems waste large amounts of heat. if there was a way to directly harvest it, almost everything in the world would be way more efficient.